This invention relates generally to sensing devices and more particularly to sensing devices which use inorganic chemical salt compositions that change color in response to changes in certain conditions such as humidity and/or temperature so that a visual representation of such conditions are indicated, normally in digital form.
A number of sensing devices have been proposed which utilize inorganic chemical salt compositions that change color in response to changes in certain conditions such as relative humidity. Examples of such devices which utilize inorganic chemical salt compositions to sense changes in relative humidity are illustrated in U.S. Pat. Nos. 2,249,867 and 3,680,364. These types of such prior art sensing devices have a common problem in that it is relatively difficult to visually perceive changes in the color of the inorganic chemical salt compositions in response to small changes in the relative humidity being sensed. Another problem associated in the manufacture of such prior art sensing devices is the difficulty in insuring that the inorganic chemical salt compositions are distributed uniformly throughout the visual area of the device to insure adequate representation of the changes in the condition. Prior art thermometers have also been suggested which utilize encapsulated organic liquid crystal compositions that change color in response to certain changes in temperature. Examples of such prior art thermometers are illustrated in Pat. Nos. 3,861,213; 3,704,625; and 3,661,142. One of the problems associated with such prior art thermometers is the difficulty in applying the organic liquid crystal compositions in the desired configuration and consistency to obtain the desired temperature response. Another problem associated with such prior art thermometers is that the organic liquid crystal compositions rapidly deteriorate when exposed to ultraviolet radiation.